The Synchronous Optical Network (SONET) was created as the standard fiber optic transmission system for long-distance telephone and data communications. SONET is most commonly described as a high bit-rate fiber-optic-based transport method that provides the foundation for linking high-speed ATM (Asynchronous Transfer Mode) switches and multiplexers and providing users with B-ISDN (Broadband—Integrated Services Digital Network) compliant services. SONET has a very detailed architecture and also comprises a series of protocols to implement this architecture. As the architecture is quite complex, this document will only describe those portions of the protocols and frame structure which are useful for explanation purposes.
First, the SONET hierarchy consists of a number of levels, organized according to data transmission speed. Each level has an optical carrier (OC) and an electrical level transmission frame structure, termed the synchronous transport signal (STS). The notation OC-N refers to the nth level of the optical carrier. The counting units are the basic 51.84 Mbps bit rate of OC-1. OC-3, for example, has a bit rate of 155.52 Mbps which is derived from the 3 times multiplier of OC-1. The STS-N notation follows that of the OC-N. For example, an STS-3 frame is sent on an OC-3 fiber link at 155.52 Mbps. The payload, a term commonly referred to, is used to indicate user data within a SONET frame.
SONET physical layer devices are often required to handle multiplexed SONET frame formats. In these frame formats, several lower-rate SONET data streams are byte-multiplexed to construct a higher-rate SONET stream. For example, it is possible to combine four STS-48 SONET frame streams (each running at 2.488 Gb/s) to create a single STS-192 SONET stream at 9.952 Gb/s by byte-interleaving: each set of four consecutive bytes within the STS-192 stream is drawn from the four STS-48 streams, taken in turn. A similar process can be used to produce an STS-192 stream from sixteen STS-12 streams at 622.08 Mb/s, or from some combination of STS-48 and STS-12 streams, and so on.
The American National Standards Institute (ANSI) T1.105-1995 standard, titled “Synchronous Optical Network (SONET)—Basic Description including Multiplex Structure, Rates and Formats,” further elaborates on the SONET frame formats and the data rates previously mentioned.
Processing such multiplexed frame streams presents a particular challenge as the data rates increase. On the receive side of the physical layer device, it is necessary to split the higher-rate SONET stream apart into the individual sub-streams and then to process each sub-stream separately. A reverse problem occurs on the transmit side. Using the traditional approach of dedicating a processing element to each individual sub-stream can result in a significant amount of hardware overhead. For example, implementing a device to process all possible valid combinations of lower-rate channels within an STS-192 would require 277 individual payload processors (1×STS-192, 4×STS-48, 16×STS-12, 64×STS-3, and 192×STS-1). Also, at any one time many of these processors would be inactive. This approach therefore leads to significant hardware inefficiencies as well.